I think a good idea would be to change the colour of failed downloads when pressing Control + J to bring up the downloads menu for failed downloads with a retry icon to be in red because right now they are the same background colour as completed and this is a huge mistake it makes it difficult to determine whats downloaded and what's not.
I don't know of any addon that can make this clear even AI couldn't find one, and i think all Firefox versions should be updated to allow this. Even firefox for Windows 7 115.34.0esr (64-bit)